normalizer_is_normalized has nullable string parameter about polyfill

[stof]

This is done because in non-strict mode, PHP accepts strings in core functions using a string typehint (triggering a deprecation in PHP 8.1+), which we cannot fully emulate in userland (that's part of the reasons why this behavior is deprecated in PHP 8.1 btw). See #330

[othercorey]

Thanks for explaining! This definitely sounds correct for pollyfill and the issue I ran into updating psalm callmap can be handled differently.